Chromatin structure and architecture. Within the cell nucleus, histone proteins package and condense DNA into chromatin. In its compact form, chromatin assumes a 30-nm fiber in vitro. Various models describe this fiber and, in collaboration with the Bai laboratory, we have utilized reconstituted nucleosome arrays to understand its structure. Using a combination of structural, scattering, and hydrodynamic methods, we showed that the 30-nm arrays reveal an ensemble of dynamic structures, consistent with numerous in vivo studies. In fact, static forms of this 30-nm fiber are only observed for inactive chromatin, as found within mature avian erythrocytes. To further our understanding of chromatin structure in vivo, we focused on the organizing protein CTCF that acts by imposing topological constraints. We have previously shown that the N- and C-terminal domains flanking the DNA binding 11 zinc fingers of CTCF are intrinsically disordered and found that many related DNA binding proteins share this property. We have characterized the physical nature of these unstructured and conserved domains. Current work focuses on the identification of high-affinity partners that bind to the N- and C-termini of CTCF and a study of the complexes formed, with the aim of understanding how CTCF, and related proteins, regulate higher-order genome organization within the eukaryotic nucleus. Macromolecular assemblies of biological interest. Hydrodynamic methods, particularly sedimentation velocity analytical centrifugation, are used to characterize important biological assemblies to obtain information on their shape, stoichiometry, and affinity of interaction. In collaboration with the Bax lab, we have focused our attention on the HIV-1 gp41 viral coat protein. Viral gp41 is composed of multiple domains and is the primary driver of the membrane fusion process during infection. Using a combination of nuclear magnetic resonance, electron paramagnetic resonance and analytical centrifugation we showed that, contrary to expectations, the transmembrane portion of gp41 adopts a monomeric alpha-helical arrangement when solubilized in bicelles. This arrangement destabilizes the cell membrane and primes it for fusion to the HIV-1 viral envelope.